In 3GPP Release 14, the concept of a lightly connected user equipment (UE) or wireless device is introduced. A lightly connected UE may look like it is in ECM connected from the perspective of the core network (e.g., an MME). The lightly connected UE may also have idle mode behavior in the radio access network (RAN) and, thus, may perform mobility between cells in the same base station or to cells on other base stations without informing the network.
The RAN and/or the base stations in the RAN have the responsibility to find a lightly connected UE when needed. For example, the RAN may need to find a lightly connected UE when user data is received by the base station or when the base station needs to transmit control plane related signaling such as a NAS message. A base station may request another base station to page the UE via a direct interface between two base stations such as the X2 interface in LTE.
In order to save battery, the UE only listens to paging at specified times derived from the UE identity index value. In LTE, when a UE is ECM_IDLE, the UE identity index value is sent to the base station from the core network in a paging message. From this information, the paging occasions are calculated. This is described in TS 36.304, which is quoted in part below:                PF, PO, and PNB are determined by following formulae using the DRX parameters provided in System Information:                    PF is given by following equation:SFN mod T=(T div N)*(UE_ID mod N)            Index i_s pointing to PO from subframe pattern defined in 7.2 will be derived from following calculation:i_s=floor(UE_ID/N)mod Ns             If P-RNTI is monitored on MPDCCH, the PNB is determined by the following equation:PNB=floor(UE_ID/(N*Ns))mod Nn″The UE_ID is the UE identity index value. The PF is the paging frame, which may be 10 milliseconds or 10 subframes in LTE. The PO is the paging occasion and identifies the particular subframe in a frame for transmission of a page. The PNB is the paging narrowband on which the UE performs the paging message reception.                        
While in a lightly connected state or inactive state, there may be a need for a base station to page the UE. Unfortunately, however, a base station triggering paging of an inactive UE does not have sufficient information to calculate when a UE camping on is listening for paging since the base station does not need the UE_ID for a UE in connected mode.